Hydroxy sulfonic acids and, more particularly, 2-hydroxyethane sulfonic acid, more commonly known as isethionic acid (ITA), have been prepared by a variety of methods over the past century. For example, the Tenth Edition of the Merck Index, at page 4960, describes ITA synthesis procedures in which ethylene is treated with a sulfonating agent such as chlorosulfonic acid and ethylene oxide with sulfur trioxide. Such methods, however, yield a product contaminated with salts such as chlorides and sulfates. Such contaminated products are undesired in a number of applications where the salt-free acid, having an amount of contaminating salts below about 0.1% is required or desired. One such application is the formation of sulfo esters, such as sulfo ethyl and sulfo propyl methacrylate, which find wide use as stabilizers in vinylidene chloride copolymers used for packaging films. Since the prior art methods are found to produce sulfonic acid compositions containing high, and usually undesirable, levels of contaminants, methods such as ion exchange chromatography have been used either to remove these impurities or to convert alkali metal isethionates, disclosed for example in U.S. Pat. No. 2,899,461 to Smith, to the acid form. However, such methods are expensive and difficult to carry out.
As a result, a number of other approaches to producing low impurity alkyl sulfonic acids have been investigated. For example, Koenig in U.S. Pat. No. 2,892,852 describes a process for producing such acids by the oxidation of organic thioethers or thioacetic acid esters with peracetic acid in acetic acid. Other processes use potassium permanganate, nitric acid or ozone as the oxidizing agent, see Asinger, Journal Praktische Chemie, (4)(2), Vol. 2, pp. 241-242 (1955). However, in all the above-cited processes, the yields achieved have proven to be only moderate and these processes have found only slight use for industrial purposes. This is also true of a process involving the electrolytic oxidation of methyl mercaptan as disclosed in U.S. Pat. No. 2,727,920. There is also described in that patent the reaction of mercaptans with nitric acid, which, however, cannot be employed for large-scale processes because of the explosive-like acceleration of the reaction velocity.
It is additionally known that hydrogen peroxide can oxidize certain mercaptans provided that a sufficient amount of carboxylic acid is also present. Consequently, it was thought that the percarboxylic acid formed from the carboxylic acid and the hydrogen peroxide was the true active agent (Showell, Journal of Organic Chemistry, Vol. 27, (1962), pp. 2853-2858). It is particularly disadvantageous in this process that it results in a product which is a mixture of alkyl sulfonic acids and carboxylic acids or percarboxylic acids from which the desired alkyl sulfonic acid is recovered in pure form only with difficulty. In the particular case of isethionic acid synthesis, techniques such as solvent extraction or distillation, which are often used to remove such compounds, are not feasible because of the solubilities and physical characteristics of isethionic acid.
Most recently, Schreyer et al. in U.S. Pat. No. 4,239,696 has disclosed a process in which alkyl sulfonic acids are prepared by the oxidation of a stoichiometric excess of an alkyl mercaptan or dialkyl disulfide, dissolved or suspended in an alkyl sulfonic acid, with hydrogen peroxide, and in the absence of percarboxylic acids. In the disclosed process, a 1 to 35 mole percent of alkyl sulfonic acid, based on the alkyl mercaptan or dialkyl disulfide used, is preferably employed. Schreyer further discloses that since no carboxylic acids are involved in this synthesis, many of the aforementioned purification problems are avoided.